A combustion engine is an engine in which the combustion of fuel and air occurs within a combustion chamber. The combustion process burns the fuel and air mixture to create a gas at high temperature. The high temperature gas creates high pressure that is then used to apply force to a piston to perform work. Because the combustion process generates a gas, the ideal gas law can be used to determine the relationship between temperature, pressure, and volume of the gas.
The ideal gas law is PV=nRT, where
P=pressure
V=volume of the gas
T=temperature
n=number of moles of gas
R=ideal gas constant
Given a constant quantity of gas, the pressure of the gas is directly related to its temperature and inversely related to its volume.
Most combustion engines in use today use a crank slider mechanism (CSM) to transfer the power from the linear motion of the piston to circular motion. The CSM includes a piston connected to a crankshaft (crank) by a connecting rod, as is shown in FIG. 1. In FIG. 1, FW is the force perpendicular to the crank and generates torque T on the crank. Fp is the force on the piston caused by the combustion process. The relationship between T, FW and Fp is shown as follows:T=FW*(stroke/2)
FW is related to Fp by the following equationFW=Fp*sin(180−β−a)
FIG. 2 is a graph showing the force FW as a percentage of force Fp as the crank rotates from angle θ at Top Dead Center (TDC) to 180 degrees at Bottom Dead Center (BDC) for a crank slider with a stroke of 4 inches and a connecting rod 6 inches in length. It can be seen from this graph that FW is equal to 0 at TDC and increases until FW is equal to Fp at 65 degrees of crank motion and then decreases until FW is again 0 at 180 degrees.
According to the ideal gas law, the force on the piston (Fp) varies with the inverse of the volume of the gas. FIG. 3 (Prior Art) is a graph showing the relationship between gas pressure and the crank angle for a typical combustion engine. In this chart, negative degrees are before TDC and positive degrees are after TDC with 0 being TDC. The pressure rises before TDC as the fuel mixture is being compressed into a smaller volume. In addition, for real systems, the fuel mixture takes a finite amount of time to burn requiring the fuel mixture be ignited before TDC. This can be seen in the graph as a change of the slope in the curve as the fuel mixture is ignited at −20 degrees before TDC. FIG. 3 (Prior Art) shows that the pressure peak occurs 5 degrees past TDC but this peak can be moved by igniting the fuel earlier or later in the combustion process. The slope of the pressure decrease after peak is driven by the amount of additional volume in the combustion chamber as the crank rotates.
FIG. 4 (Prior Art) is a graph showing two curves of pressure versus crank angle. The first curve (solid line) has the pressure peak at 5 degrees after TDC and the second curve (dashed line) has the pressure peak at 20 degrees after TDC. The graph shows that the crank angle at which the power peak occurs can be changed but such comes at a price, as the volume of the gas increases the later the fuel mixture is ignited, resulting in a lower overall peak value.
FIG. 5 (Prior Art) is a graph comparing available pressure from the combustion process to the pressure that is converted to do work. The “Pressure Available” curve is derived from the ideal gas law with temperature constant and 100 being the force available at TDC. The pressure available drops as the crank rotates and the volume of the combustion chamber expands. The “CSM” curve is derived from multiplying the CSM percentage of force converted to work times the force available. FIG. 5 shows that close to TDC, there is a great deal of pressure but very little of it is converted to work, which is shown as the large gap between the two curves at the lower crank angles. This comparison shows that at the pressure peak, 0% of the force is used to perform work and by the time 100% of the pressure is converted into work, the pressure is 25% of its peak. Because of this, a conventional crank slider mechanism only converts approximately one half of the available pressure into work.
Engines not having a conventional crank slider mechanism have been proposed in U.S. Pat. No. 6,684,828 to Ushijima; U.S. Pat. No. 7,213,563 to Yaguchi; U.S. Pat. No. 7,992,529 to Kobayashi; U.S. Pat. No. 8,011,343 to Kobayashi; U.S. Pat. No. 8,100,098 to Takahashi; U.S. Pat. No. 8,161,922 to Watanabe; U.S. Pat. No. 8,171,899 to Watanabe; U.S. Pat. No. 8,281,764 to Gurler and U.S. Pat. No. 8,327,819 to Voegeli.